Method of controlling torch height in plasma cutting

ABSTRACT

The present invention discloses a method of controlling torch height in plasma cutting arranged to monitor the cutting speed so that the optimum torch height is maintained and an excellent cutting quality is obtained even if the cutting speed changes. To this end, a base metal (2) is cut by a plasma arc (3) while a torch (1) is maintained at an optimum height (hc) from the base metal (2), and arc voltage (Vi) is read several times (i=1 to n) after the arc voltage becomes steady, their mean arc voltage (Vb) is calculated, a cutting speed (Fa) is read at the time of steady cutting, the means arc voltage (Vb) is corrected by this cutting speed (Fa) to obtain a target arc voltage (Vc) and the optimum height (hc) is maintained by this target arc voltage (Vc).

TECHNICAL FIELD

The present invention relates to a method of controlling the height of atorch in plasma cutting, and, more particularly, to a method ofmaintaining a torch at its optimum torch height by monitoring thecutting speed so as to achieve quality cutting even if the cutting speedchanges.

BACKGROUND ART

Hitherto, the plasma cutting machine has been mounted on an XY table orthe like and adapted to a method of controlling the torch height inplasma cutting in order to obtain quality cutting, the method beingarranged to utilize a fact that "torch height h is in proportion to arcvoltage Va" at a predetermined torch movement speed (hereinafter calleda "cutting speed") in such a manner that the arc voltage Va isomonitoredso that the torch is maintained at its optimum height hc (refer to, forexample, DE2706232C3).

Specifically, as shown in FIG. 7, an assumption is made that the arcvoltage generated when the torch is positioned at the optimum torchheight hc is a target arc voltage Vci (SO), the torch is set at theaforesaid optimum cutting height hc from a base metal (S5), and then thebase metal is cut. Furthermore, after the arc voltage Va becomes steady(S6), the arc voltage Va is read (S93), and this arc voltage Va and theaforesaid target arc voltage Vc1 are subjected to a comparison (S10). IfVa>Vc1, the torch is lowered (S101). If Va=Vc1, the existing torchheight is retained (S102). If Va<Vc1, the torch is moved upwardly(S103). As result, the torch is maintained at its optimum torch heighthc.

The aforesaid optimum torch height hc is a value which can be determinedpredictively by collectively considering various factors such as thethickness and the material of the base metal, the diameter of the nozzleof the torch, the cutting speed, and the torch height. The target arcvoltage Vc1 obtained from the optimum height hc has been set into thecontrol system of the subject plasma cutting machine as a fixed value.

Observing the actual cut surface, if the torch height is constant at itsoptimum, the quality of cutting is also constant (in other words, if thetorch height is changed, the quality of cutting deteriorates). With theaforesaid conventional method, the cutting speed is changed at the timeof cutting a corner, although an assumption is made that a constantcutting speed is maintained. Accordingly, the optimum torch height isalso changed, causing the quality of cutting the corner to deteriorateas described in detail below.

FIG. 8 is a characteristic graph which illustrates the results ofmeasurements of cutting speeds (axis of abscissa) and arc voltages (axisof ordinate) with respect to various torch heights h1 to h5 (h1<h5) . Asshown in FIG. 8, an increase in the cutting speed Fa will lower the arcvoltage Va (hereinafter called an "in inverse proportion") at each torchheight h1 to h5. The reason f or this lies in the fact that an increasein the cutting speed causes the main positive point to come closer tothe torch.

In a case where the predicted optimum torch height is h1 and thepredicted optimum cutting speed is FL, the arc voltage Va is Vc1 (forconvenience in description, an assumption is made that the arc voltageVa=the target arc voltage Vc1) . If the cutting speed is raised (FL→FH),the arc voltage Va is lowered (Vc1 →VL). Since the target arc voltageVc1 is a fixed value, the aforesaid values are subjected to acomparison, resulting in VL<Vc1. Hence, the torch is raised to heighth3. That is, there arises a problem in that the torch height has to bechanged because the cutting speed has been changed.

In order to overcome the aforesaid problems experienced with theconventional technology, an object of the present invention is toprovide a method of controlling the torch height in plasma cutting whichis capable of overcoming the deterioration of cutting quality due to achange in the cutting speed.

SUMMARY OF THE INVENTION

In a first aspect, a method of controlling torch height in plasmacutting according to the present invention comprises the steps of:setting a torch at an optimum cutting height hc from a base metal sothat the base metal is cut by using a plasma arc; reading arc voltagesVi several times (i=1 to n) after the arc voltage becomes steady;calculating an average arc voltage Vb of the thus read arc voltages Vi;reading the cutting speed Fa at a steady cutting operation; correctingthe average arc voltage Vb by the cutting speed Fa to obtain a targetarc voltage Vc; and maintaining the optimum torch height hc inaccordance with the target arc voltage Vc.

In a second aspect, the present invention is characterized by a methodof controlling torch height in plasma cutting in which the arc voltageat an optimum cutting height hc is set as a target arc voltage vcl, atorch is set to the optimum cutting height hc from a base metal, thebase metal is cut by using a plasma arc, an arc voltage Va is read afterthe arc voltage becomes steady, and the arc voltage Va and the targetarc voltage Vc1 are subjected to a comparison, so that the optimum torchheight hc is maintained, the method of controlling torch height inplasma cutting comprising the steps of: reading a cutting speed Fa; andcorrecting the target arc voltage VC1 by a changed voltage ΔV which isdetermined in accordance with the cutting speed Fa.

In a third aspect, the present invention is characterized by a method ofcontrolling torch height in plasma cutting, comprising the steps of:setting a torch at an optimum cutting height hc from a base metal sothat the base metal is cut by using a plasma arc; reading arc voltagesVi several times (i=1 to n) after the arc voltage becomes steady;calculating an average arc voltage Vb of the thus read arc voltages Vi;reading a cutting speed Fa at a steady cutting operation; correcting theaverage arc voltage Vb by a changed voltage ΔV which is determined inaccordance with the cutting speed Fa to obtain a target arc voltage Vc;then reading the arc voltage Va; subjecting the arc voltage Va and thetarget arc voltage Vc to a comparison; lowering the torch if Va>Vc;retaining the existing torch height if Va=Vc; and raising the torch ifVa<Vc, so that the optimum torch height hc is maintained.

In a fourth aspect, the present invention is characterized by a methodof controlling torch height in plasma cutting, comprising the steps of:setting a torch at an optimum cutting height hc from a base metal sothat the base metal is cut by using a plasma arc; reading an arc voltageV1 after the arc voltage becomes steady; retroactively reading each arcvoltage (Vi where i=2 to n) to n-1 times so as to calculate an averagearc voltage Vb of the read arc voltages; reading a cutting speed F at asteady cutting operation; correcting the average arc voltage Vb by achanged voltage Δv which is determined in accordance with the cuttingspeed Fa to obtain a target arc voltage Vc; then reading the arc voltageVa; subjecting the arc voltage Va and the target arc voltage Vc to acomparison; lowering the torch if Va>Vc; retaining the torch height ifVa=Vc; and raising the torch if Va<Vc, so that the optimum torch heighthc is maintained.

In a fifth aspect, the present invention is characterized by a method ofcontrolling torch height in plasma cutting, comprising the steps of:setting a torch at an optimum cutting height hc from a base metal sothat the base metal is cut by using a plasma arc; reading an arc voltageV01 and its cutting speed F01 after the arc voltage becomes steady;calculating the arc voltage V01 as a function V11= g(F) of the cuttingspeed F01; calculating a reference arc voltage Δ10 to be generated at areference cutting speed F0 individually determined in the function V11;reading each reference arc voltage (V10 where i=2 to n) for each timeretroactively stored to the (n-1) th time; calculating their averagereference arc voltage Vb0; calculating a function V=g(F), which is afunction at the reference cutting speed F0, in accordance with theaverage reference arc voltage Vb0; reading a cutting speed Fa at asteady cutting operation; calculating an arc voltage in the function Vat the cutting speed Fa to obtain a target arc voltage Vc; then readingan arc voltage Va; subjecting the arc voltage Va and the target arcvoltage Vc to a comparison; lowering the torch if Va>Vc; keeping thetorch height if Va=Vc; and raising the torch if Va<Vc, so that theoptimum torch height hc is maintained.

In a sixth aspect, the present invention is characterized by a structurearranged in such a manner that the process of reading the arc voltage(Va) is simultaneously performed in the process of reading the cuttingspeed (Fa) according to the second, the third, the fourth, and the fifthaspects.

Each of the aforesaid structures is arranged in such a manner that thearc voltage Va is monitored by utilizing the fact that "the torch heightis in proportion to the arc voltage at the same cutting speed", and thecutting speed Fa is monitored by utilizing the fact that "the arcvoltage Va (that is, the torch height) is substantially in inverseproportion to the cutting speed Fa". As a result, even if the cuttingspeed Fa has been changed, the optimum torch height hc is maintained.Specifically, the structure is arranged in such a manner that the targetarc voltage Vc is corrected by the cutting speed Fa. Therefore, even ifthe cutting speed has been changed, the optimum torch height hc can bemaintained, and hence an excellent cutting quality can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart for use in an embodiment according to a firstaspect of the present invention;

FIG. 2 is flow chart for use in an embodiment according to a secondaspect of the present invention;

FIG. 3 is a flow chart for use in an embodiment according to a thirdaspect of the present invention;

FIG. 4 is a flow chart for use in an embodiment according to fourthaspect of the present invention;

FIG. 5 is a flow chart for use in an embodiment according to fifthaspect of the present invention;

FIG. 6 is a schematic view which illustrates a plasma cutting robot onwhich the embodiment according to any one of the first fifth aspects ofthe present invention is mounted;

FIG. 7 is a flow chart which illustrates a conventional method; and

FIG. 8 is a graph which illustrates the relationship between the cuttingspeed and the arc voltage at a predetermined torch height.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will now be described withreference to FIGS. 1 to 6. Each embodiment is adapted to a plasmacutting robot shown in FIG. 6. The robot 5 comprises a rotary device 52disposed on a base frame 51, a boom 53 mounted on it, an arm 55connected to the leading portion of the boom 53 with a pin and arrangedto be elevated by a hydraulic actuator 54, a hand 56 joined to theleading portion of the arm 55 in a universal manner, and a controller57. A torch 1 is fastened to the aforesaid hand 56. Furthermore, a smokedischarge equipment 6 is disposed below the torch 1. A base metal 2 isso placed on a retainer 61 disposed on the smoke discharge equipmentthat the base metal 2 faces the torch 1.

The plasma cutting operation is performed in such a manner that arcvoltage Va generated by a power source 41 between a torch electrode (-)and the base metal 2 (+) is used to convert an operation gasindividually supplied to a nozzle disposed at the leading portion of thetorch 1 into a plasma arc 3, and the plasma arc 3 and an operating gasindividually supplied to the aforesaid nozzle to surround the plasma arc3 are jetted along a cutting line of the base metal 2 so that the basemetal 2 is melted and blown away.

Each of the embodiments is stored by a controller 42 comprising amicrocomputer so that arc voltage Va and cutting speed Fa are read,stored and calculated, and the results of the calculations are outputtedto a torch drive system of the robot 5. The term "height" for use intorch height h means the "distance" between the torch 1 and the basemetal 2 as can be understood from a fact that the aforesaid plasmacutting robot performs the three-dimensional cutting (for example,diagonal cutting, horizontal cutting and vertical cutting) operation.The cutting speed Fa is obtained by reading a set value of the cuttingspeed stored by the robot controller 57 in the embodiment to bedescribed below. However, it may be read by means of a speed sensordisposed outside individually.

FIG. 1 is a flow chart for use in the first embodiment according to thepresent invention. The controller lowers the torch 1 to a predeterminedheight ha from the base metal 2 (S1). When a limit switch disposedadjacent to the torch 1 is switched ON, the controller reads the heightha to obtain a first reference (S2). Then, it lowers the torch 1 topiercing height hb so as to read the height hb which is then made to bea second reference (S3), and then piercing is performed (S4).

The piercing operation is an operation for forming a through hole at astart point in a case where the-start point at which the cuttingoperation is commenced is in the surface of the base metal at a locationremote from the edges of the base metal. By performing the piercingoperation, molten materials, which cause a double-arc or breakage of thenozzle and the torch to take place, can be blown out. Therefore, thepiercing height hb is always made to be higher than the torch height hcto be realized at the time of steady cutting. In another case where thecutting start point is located in the edge portion of the base metal orin the case where a thin base metal is machined, the piercing processstep (S4) can be omitted. The aforesaid process steps (S1 to S4) arecommonly performed in each of the embodiments relating to the first tofifth aspects, and therefore their descriptions are omitted in theaforesaid embodiments.

The embodiment according to the first aspect is arranged as follows:after the aforesaid preliminary process steps (S1 to S4), the torch 1 isfurther lowered to the optimum torch height hc (S5) so that the basemetal 2 is cut by the plasma arc 3. After the arc voltage becomes steady(S6), arc voltage Vi is read n times, for example ten times (S7), the,average arc voltage Vb is calculated (S8), the cutting speed Fa is readat the time of the steady cutting operation, the aforesaid average arcvoltage Vb is corrected by the cutting speed Fa so as to make the resultof the correction to be the target arc voltage Vc (S9), and the optimumtorch height hc is maintained by using the aforesaid target arc voltageVc (S10). The last two process steps (S9 and S10) are looped until theplasma cutting operation is completed (S13). After the cutting operationhas been completed (S11), the plasma arc 3 is turned off (S12), and thetorch 1 is moved upwardly (S14). Thus, the overall process is completed.The aforesaid completion process steps (S11 to S14) are commonlyperformed in the embodiments according to the second to fifth aspects,and therefore their descriptions are omitted in the aforesaidembodiments.

The first aspect of the present invention (also the third embodiment) issuitable to cut a large portion in one base metal.

FIG. 2 is a flow chart for use in an embodiment according to the secondaspect of the present invention. First, the arc voltage generated whenthe torch 1 is at the optimum torch height hc is set as the initialtarget arc voltage Vc1 (S0) , the torch 1 is set to the optitaum cuttingheight hc from the base metal 2 (S5), and the base metal 2 is cut byusing the plasma arc 3. Furthermore, after the arc voltage becomessteady (S6), the cutting speed Fa is inputted (S91), then the aforesaidinitial target arc voltage Vc1 is corrected by changed voltage ΔVobtained in accordance with the cutting speed Fa so as to make theresult to be the target arc voltage Vc (S92), the arc voltage Va is read(S93), the arc voltage Va and the target arc voltage Vc are subjected toa comparison (S10). If Va>Vc, the torch 1 is lowered (S101). If Va=Vc,the torch height is retained as it is (S102). If Va<Vc, the torch ismoved upwardly (S103), so that the optimum torch height hc is maintained(S10).

In this embodiment, the changed voltage ΔV is outputted in accordancewith the following process. The controller 41 has previously stored thechanged voltage ΔV which depends upon each cutting speed F_(am) at theoptimum torch height hc as specific values. The changed voltage ΔV atthe stored cutting speed F_(am) which coincides with the read cuttingspeed Fa is outputted. Further in detail, the cutting speed Fa isrounded after it has been read so that it is made to serve as thecutting speed F_(am). The process of outputting the changed voltage ΔVis commonly used in the embodiments according to the third and thefourth aspects of the invention and therefore its description is omittedin these embodiments.

Although the target arc voltage Vc1 is the target arc voltage in thecase of the conventional technology as can be seen from its sign, thesecond aspect of the present invention is preferable and convenient whenit is applied to the case where a determination is made that anexcessively improved cutting quality is made in the first, and the thirdto the fifth aspects of the present invention and another determinationis made that an unsatisfactory cutting quality is made in theconventional technology because it is corrected by the cutting speed Va.If the optimum torch height hc is further precisely inspected, cuttingquality equivalent to that obtainable from the first, and the third tothe fifth aspects of the present invention can, of course, be obtained,and therefore, the second aspect is the base of a series of the aspectsof the present invention.

FIG. 3 is a flow chart for use in the embodiment according to thirdaspect of the present invention, wherein the torch 1 is set to apredicted optimum torch height hc from the base metal 2 (S5), the basemetal 2 is cut by using the plasma arc 3, and after the arc voltagebecomes steady (S6) the arc voltage Vi is read n times, for example tentimes (S81), the average arc voltage Vb is calculated (S82), the cuttingspeed Fa to be realized at the steady cutting operation is read (S91),the aforesaid average arc voltage Vb is corrected by the changed voltageΔV in accordance with the cutting speed Fa to obtain the target arcvoltage Vc (S92), then the arc voltage Va is read (S93), and the arcvoltage Va and the aforesaid target arc voltage Vc are subjected to acomparison (S10). If Va>Vc, the torch 1 is lowered (S101). If Va=Vc, thetorch height is retained (S102). If Va<Vc, the torch 1 is moved upwardly(S103). As a result, the predetermined optimum torch height hc ismaintained (S10).

FIG. 4 is a flow chart for use in the embodiment according to the fourthaspect of the present invention, wherein the torch 1 is set to thepredicted optimum height h from the base metal 2 (S5), the base metal 2is cut by using the plasma arc 3 and, after the arc voltage becomessteady (S6), the arc voltage V1 is read (S81), and each of the arcvoltages V2 to V9 to the ninth time is read retroactively and theiraverage arc voltage Vb is calculated (S82), the cutting speed F to berealized at the time of the steady cutting operation is read (S91), theaforesaid average arc voltage Vb is corrected by the changed voltage ΔVin accordance with the cutting speed F to obtain the target arc voltageVc (S92), then the arc voltage Va is read (V93), and the arc voltage Vaand the aforesaid target arc voltage Vc are subjected to a comparison(S10). If Va>Vc, the torch is lowered (S101). If Va=Vc, the torch heightis retained (S102). If Va<Vc, the torch 1 is moved upwardly (S103). As aresult, the optimum torch height hc is maintained (S10).

The fourth aspect is different from the first and the third embodimentsin that its advantage is obtainable when the piercing, the torchdownward movement, the cutting, the torch upward movement, the torchshifting, the torch downward movement, the cutting, the torch upwardmovement process steps are continuously performed in a case where thereis a plurality of shapes to be formed by the cutting operations in onebase metal. That is, the average arc voltage Vb as the subject cuttinginformation is formed by including a predetermined number of times (n-1times) of the previous information items Vi (the aforesaid arrangementis commonly employed in the fifth aspect of the present invention to bedescribed later).

In a case where the number of times of reading the arc voltages Vi isless than a predetermined times n (that is, the number of times ofstarting the cutting operations does not reach the predetermined times),the average arc voltage Vb is calculated in such a manner that the arcvoltage V1 read at the first time is stored by all of the insufficienttimes of reading. For example, in a case where the number of readingtimes is three, the subject arc voltage V1 is stored for the subjectreading, the previous voltage V2 is stored for the previous reading, andeight arc voltage V3 before two times are stored from the reading beforetwo times to the reading before nine times. From these read arcvoltages, the average arc voltage Vb is calculated. Although amultiplicity of methods may be available, the embodiment of the fifthaspect employs the aforesaid method (however, cutting informationaccording to the fifth aspect of the present invention is V10 in placeof the aforesaid information Vi).

FIG. 5(A) is a flow chart for use in the fifth aspect of the presentinvention. FIGS. 5(B) to 5(D) are graphs for describing the processesshown in FIG. 5(A), where the relationship between the cutting speed F(axis of abscissa) and the arc voltage V (axis of ordinate) at anoptimum torch height is shown. The base metal 2 is cut by using theplasma arc 3, and after the arc voltage becomes steady (S6) the arcvoltage V01 and the cutting speed F01 corresponding to it are read(S811), the aforesaid arc voltage V01 is obtained from a functionV11=g(F) of the aforesaid cutting speed F01 (S812), the reference arcvoltage V10 at the time of reference cutting speed F0 individuallydetermined in accordance with the aforesaid function V11 is calculated(S813), as well as each reference arc voltage V10 (i=2 to n) stored foreach time retroactively to, for example, the ninth time is read andtheir average reference arc voltage Vb0 is calculated (S82), furthermorethe aforesaid average reference arc voltage Vb0 is calculated as thefunction V=g(F) which is the function at the time of the aforesaidreference cutting speed F0 (S821), the cutting speed Fa to be realizedat the steady cutting operation (S91), the arc voltage in the function Vat the aforesaid cutting speed Fa is calculated to obtain the target arcvoltage Vc (S92), then the arc voltage Va is read (S93), and this arcvoltage Va and the aforesaid target arc voltage Vc are subjected to acomparison (S10). If Va>Vc, the torch is lowered (S101). If Va=Vc, thetorch height is retained (S102). If Va<Vc, the torch 1 is moved upwardly(S103). As a result, the optimum torch height hc is maintained (S10).

Although the changed voltage ΔV is used in the second to the fourthaspects, the function V11=g(F) is employed in the fifth aspect. Furtherin detail, the profiles of the graph can be assumed to be the samealthough they are different from one another in each intercept (that is,the parallel translation can be made in the direction of the axis ofordinate) as can be seen from the foregoing graph shown in FIG. 8. Thatis, the arc voltage Va is the function of the cutting speed Fa.Incidentally, the necessity for the aforesaid function to be acomplicated function such as a multidimensional function can beeliminated. For example, it can be formed by a composite functioncomposed of two or three types of linear functions having differentinclinations and by only specifying the cutting speed range to which theaforesaid two or three types of the linear functions are adapted.

Therefore, as shown in FIG. 5(B), in process steps (S811 and S812), theread arc voltage V01 and the cutting speed F01 are used to calculatefunction V11=g(F) (V01=g(F01)) which includes them. In process step(S813), the reference arc voltage V10= g (F0) at the time of thereference cutting speed F0 calculated individually is calculated inaccordance with the aforesaid function V11. This arc voltage V10 isstored until the nine forward cutting operations. The reason why thereference cutting speed F0 is determined lies in the fact that thecalculation and storage efficiencies are improved in executing theprogram by causing the controller 41 to store only basic calculationfactors such as the basic function, basic data and the like.

In process step (S82), the average reference arc voltage Vb0 of thereference arc voltage V10 and the reference arc voltages V20 to V90retroactively stored to the ninth time is calculated. The reason forthis lies in that the subject cutting must be made uniform by thedispersion for the forward nine times of the cutting operations.Incidentally, the process of storing the reference arc voltage V10 andthat of reading the reference arc voltages V20 to V90 have beendescribed previously in the description about the fourth aspect of thepresent invention.

In process step (S821), as shown in FIG. 5(C), the average reference arcvoltage Vb0 and the reference cutting speed F0 are used to calculate acomposite function V=g(F) (Vb0= g(F(0)), which includes them, by thesame process performed in process steps (S811 and S812).

Step (S92) is a process in which the cutting speed Fa read in processstep (S91) is applied to the aforesaid composite function V=g(F) toobtain the target arc voltage Vc=g(Fa). Then, the arc voltage Va read instep (S93) and the target arc voltage Vc are subjected to a comparisonin step (S10) so that the torch height is slightly moved in accordancewith the result of the comparison.

That is, when the cutting speed Fa is read into the target arc voltagefunction V corresponding to the optimum torch height hc, the target arcvoltage Vc is immediately calculated. Then, when the arc voltage Va isread, the cutting speed Fa is subjected to a comparison with this arcvoltage Va. Since Va>Vc as shown in FIG. 5(D), the actual torch heightis higher than the optimum torch height hc. Therefore, the optimum torchheight hc can be maintained by lowering the torch 1. Hence, an excellentcutting quality which is not affected by the cutting speed Fa can beassuredly realized.

In each embodiment according to the second to the fifth aspects of thepresent invention, similar results can, of course, be realized even ifthe arc voltage Va is read (S93) simultaneously with reading the cuttingspeed Fa (S91).

According to the method of controlling the torch height in plasmacutting which includes the structure for monitoring the cutting speed,the optimum torch height can be maintained even if the cutting speedchanges. Therefore, an excellent cutting quality can be maintained.

Another embodiment of the present invention will now be described.

First, the actual arc voltage V generated by the plasma torch 1 and itsreference arc voltage V0 to be generated at the time of the referencecutting speed F0 outputted by a reference voltage setter are subjectedto a comparison (V-V010=ΔV). The deviation voltage ΔV is converted inaccordance with a torch height correction reference table previouslystored by a correction value converter so as to be outputted ascorrection command value±Δh to the robot 5 so that the plasma torch 1 isslightly moved. It should be noted that the robot 5 feeds back change=Δfof the reference speed F0 to the reference voltage setter.

Then, linear function V1 is, under predetermined conditions, set in acoordinate system composed of the actual torch heights h (axis ofabscissa) and their arc voltages V (axis of ordinate). The predeterminedconditions according to this embodiment are as follows: the referencecutting speed F0 is 1.0 m/min, the nozzle diameter φc is 0.6 mm, thework material ρc is a SS material, and the work thickness tc is 3.2 mm.The linear function V1 under the aforesaid conditions can be expressedby the following equation:

    V.sub.1 =4.7 h+111

Assuming that conditions are not the same, its linear function exhibitssimilar characteristics in the aforesaid coordinate system although theposition and the inclination becomes different to a certain degree.

Then, linear function V2 corresponding to a predetermined work materialφc is set in the coordinate system composed of the actual cutting speedFa (axis of abscissa) and the actual arc voltage V (axis of ordinate).Even if the other conditions are changed, the inclination α of thelinear function V2 is the same, which is α=-10 in this embodiment.However, if the cutting speed Fa exceeds a certain value, the arcvoltage V becomes constant, which is, according to this embodiment, 2.0m/min.

Then, the coordinates of the arc voltage V (which is 120V in thisembodiment) of the linear function V1 at the optimum torch height hc(which is 2.0 mm in this embodiment) is made to be the reference voltagein the subject plasma cutting operation.

Finally, in the coordinate system for the linear function V2, linearfunction Vcc passing through point Pcc the coordinates of which are thereference voltage Vc (120V) and the reference cutting speed F0 (1.0u/min) and having the inclination α (-10) of the aforesaid linearfunction V2 is set. That is, ##EQU1##

Then, the reference voltage Vc which includes the change±ΔV of the arcvoltage which corresponds to the change±Δf from the reference cuttingspeed F0 is outputted.

According to this embodiment, it can be considered that only the cuttingspeed is changed the operation of plasma-cutting the same work.Therefore, a reference voltage, which has been corrected by the cuttingspeed, can be automatically outputted.

INDUSTRIAL APPLICABILITY

The present invention is effective as a method of controlling the torchheight in plasma cutting capable of overcoming the problem of thecutting quality due to the change of the cutting speed, and is adaptedto plasma welding.

What is claimed is:
 1. A method of controlling torch height in plasmacutting, comprising the steps of: setting a plasma torch (1) at anoptimum cutting height (hc) from a base metal (2) and generating an arcvoltage (Va) between the base metal (2) and an electrode of the plasmatorch (1) so that said base metal (2) is cut by the resulting plasma arc(3); reading a value (Vi) of said arc voltage (Va) several times (i=1 ton) after said arc voltage (Va) becomes steady; calculating an averagearc voltage (Vb) of the thus read values (Vi) of arc voltage (Va);reading a cutting speed (Fa) at a steady cutting operation; correctingsaid average arc voltage (Vb) by said cutting speed (Fa) to obtain atarget arc voltage (Vc); and maintaining said optimum torch height (hc)by said target arc voltage (Vc).
 2. A method of controlling torch heightin plasma cutting in which an arc voltage at an optimum cutting height(hc) is set as a target arc voltage (Vc1), a plasma torch (1) is set tosaid optimum cutting height (hc) from a base metal (2) and an arcvoltage (Va) is generated between the base metal (2) and an electrode ofthe plasma torch (1) so that said base metal (2) is cut by the resultingplasma arc (3), said arc voltage (Va) is read after said arc voltage(Va) becomes steady, and the thus read arc voltage (Va) and said targetarc voltage (Vc1) are subjected to a comparison, so that said optimumtorch height (hc) is maintained, said method of controlling torch heightin plasma cutting comprising the steps of: reading a cutting speed (Fa);and correcting said target arc voltage (Vc1) by a changed voltage (ΔV)which is determined in accordance with said cutting speed (Fa).
 3. Amethod of controlling torch height in plasma cutting, comprising thesteps of: setting a plasma torch (1) at an optimum cutting height (hc)from a base metal (2) and generating an arc voltage (Va) between thebase metal (2) and an electrode of the plasma torch (1) so that saidbase metal (2) is cut by the resulting plasma arc (3); reading a value(Vi) of said arc voltage (Va) several times (i=1 to n) after said arcvoltage (Va) becomes steady; calculating an average arc voltage (Vb) ofthe thus read values (Vi) of arc voltage (Va); reading a cutting speed(Fa) at a steady cutting operation; correcting said average arc voltage(Vb) by a changed voltage (ΔV) which is determined in accordance withsaid cutting speed (Fa) to obtain a target arc voltage (Vc); thenreading said arc voltage (Va); subjecting the thus read arc voltage (Va)and said target arc voltage (Vc) to a comparison and, as a result ofsaid comparison:(1) lowering said torch (1) if Va>Vc; (2) retaining thetorch height if Va=Vc; and (3) raising said torch (1) if Va<Vc, so thatsaid optimum torch height (hc) is maintained.
 4. A method of controllingtorch height in plasma cutting, comprising the steps of: setting aplasma torch (1) at an optimum cutting height (hc) from a base metal (2)and generating an arc voltage (Va) between the base metal (2) and anelectrode of the plasma torch (1) so that said base metal (2) is cut bythe resulting plasma arc (3); reading a value (Vi) of said arc voltage(Va) after said arc voltage (Va) becomes steady; retroactively readingeach value (Vi where i=2 to n) of said arc voltage (Va) to n-1 times soas to calculate an average arc voltage (Vb) of the thus read values (Vi)of said arc voltage (Va); reading a cutting speed (Fa) at a steadycutting operation; correcting said average arc voltage (Vb) by a changedvoltage (ΔV) which is determined in accordance with said cutting speed(Fa) to obtain a target arc voltage (Vc); then reading said arc voltage(Va); subjecting the thus read arc voltage (Va) and said target arcvoltage (Vc) to a comparison and, as a result of said comparison;(1)lowering said torch (1) if Va>Vc; (2) retaining the torch height ifVa=Vc; and (3) raising said torch (1) if Va<Vc, so that said optimumtorch height (hc) is maintained.
 5. A method of controlling torch heightin plasma cutting, comprising the steps of: setting a plasma torch (1)at an optimum cutting height (hc) from a base metal (2) and generatingan arc voltage (Va) between the base metal (2) and an electrode of theplasma torch (1) so that said base metal (2) is cut by the resultingplasma arc (3); reading an arc voltage (V01) and its cutting speed (F01)after said arc voltage (Va) becomes steady; calculating said arc voltage(V01) as a function V11=g(F) of said cutting speed (F01); calculating areference arc voltage (V10) to be generated at a reference cutting speed(F0) individually determined in said function (V11); reading eachreference arc voltage (V10 where i=2 to n) for each time retroactivelystored to the (n-1)th time; calculating an average reference arc voltage(Vb0); calculating a function V=g(F), which is a function at saidreference cutting speed (F0), in accordance with said average referencearc voltage (Vb0); reading a cutting speed (Fa) at a steady cuttingoperation; calculating an arc voltage in said function (V) at saidcutting speed (Fa) to obtain a target arc voltage (Vc); then reading anarc voltage (Va); subjecting the thus read arc voltage (Va) and saidtarget arc voltage (Vc) to a comparison and, as a result of saidcomparison;(1) lowering said torch (1) if Va>Vc; (2) retaining the torchheight if Va=Vc; and (3) raising said torch (1) if Va<Vc, so that saidoptimum torch height (hc) is maintained.
 6. A method of controllingtorch height in plasma cutting according to claim 2, wherein the step ofreading said arc voltage (Va) is simultaneously performed with the stepof reading said cutting speed (Fa).
 7. A method of controlling torchheight in plasma cutting according to claim 3, wherein the step ofreading said arc voltage (Va) is simultaneously performed with the stepof reading said cutting speed (Fa).
 8. A method of controlling torchheight in plasma cutting according to claim 4, wherein the step ofreading said arc voltage (Va) is simultaneously performed with the stepof reading said cutting speed (Fa).
 9. A method of controlling torchheight in plasma cutting according to claim 5, wherein the step ofreading said arc voltage (Va) is simultaneously performed with the stepof reading said cutting speed (Fa).